The present invention relates to automatic voltage regulation and, more particularly, to voltage control for synchronous generators.
Synchronous generators are generally used to produce alternating current (AC) power, such as the 120 volts AC (VAC) at a frequency of 60 Hertz (Hz) that is the standard in much of North America. Synchronous generators typically feature precise control of voltage, frequency, and power, which is achieved through the use of voltage regulators. FIG. 1 is a simplified circuit diagram illustrating an example of a synchronous generator 10. Synchronous generator 10 is composed of a stationary armature winding, stator 12, with multiple wired connected in series or parallel to obtain a desired voltage for the output voltage VGEN of the generator. The armature winding is typically placed into a slotted laminated steel core. The generator also includes a rotor having rotor winding 14, which is fed with a DC field current to produce an electromagnetic field that rotates with the rotor to produce a revolving field with a strength that is proportional to the field current.
A mutual flux develops across an air gap between the rotor and stator, which interact to produce an electro-magnetic force (EMF). As magnetic flux developed by the DC field poles of the rotor winding 14 crosses the air gap of the stator windings 12, a sinusoidal voltage is produced and output by the generator as VGEN. The magnitude of the AC voltage VGEN output by the generator is controlled by the amount of DC excitation current that is supplied to the field winding 14 of the rotor. The frequency of the voltage developed by generator 10 depends upon the speed of the rotor, which is also affected by the excitation current supplied to the rotor field winding 14. The frequency of the generator will typically be influenced by variations or transients of the load on the generator. For example, a sudden increase in the load may cause a reduction of the generator's frequency.
An Automatic Voltage Regulator (AVR) 16 is a device that controls the output voltage of a synchronous electric generator to keep the output voltage constant as load changes. This is achieved by controlling the Field Current IFIELD in the field winding 14 of the rotor as a function of the generator's output voltage. AVR 16 controls IFIELD by controlling the field voltage VFIELD output to power section 18, which is generally a relatively high current device. The field winding 14 of the rotor can be connected directly to the AVR 16 and power section 18 by sliprings and brushes, but another approach is brushless excitation, wherein the field winding 14 is fed by an exciter thus eliminating the brushes.
AVRs are typically used to control the field or excitation current to the rotor in order to maintain the output voltage of the generator at a constant frequency and voltage. A typical automatic voltage regulator (AVR) monitors the output voltage VGEN of the generator and modulates the excitation current IFIELD to rotor field winding 14 in response to variations in frequency and voltage level of VGEN, which will be affected by variations in load 20 coupled to the generator output.
The aim of AVR design is to maximize performance for a given Prime Mover/Exciter/generator configuration. One of ordinary skill in the art would generally understand this to mean (1) control the output voltage within a small deviation range (e.g. less than 3%) of the nominal voltage on a steady state condition and (2) respond rapidly to changing loads, i.e the transient response is designed in a way that protects the load connected to it and makes good use of the prime/mover/generator being controlled. Often the AVR will be required to start up from residual magnetism (i.e. there is no separate power supply from which the AVR can excite the rotor winding in order to start the generator function).
There are generally four main performance considerations that are normally addressed in AVR design in order to control the generator output voltage under varying load conditions. A first design consideration is to adjust the generator output voltage within a given range (usually +/−10% around a nominal voltage, i.e. 120 VAC line to ground) and maintain the selected voltage in spite of load changes. In steady state conditions, most modern AVRs will maintain the voltage within 3% regulation or better from no load to full load. Some modern AVRs can maintain 1% regulation, but, in practice, anything below about 2% provides no significant benefit since electric utilities voltage regulation can often change as much as 5% and most loads are rated to operate properly within a range of +/−10% of the nominal line voltage level.
A second AVR design consideration is regulating under heavy transient loads to minimize the initial voltage drop. FIG. 2 is a graph illustrating a typical transient response of a conventional AVR in response to the introduction of a heavy transient load, such as the activation of a motor. An AVR is generally expected to minimize the initial voltage drop and then recover to steady state regulation within 3% or better of specified voltage output from no load to full load. A sub-transient voltage drop period immediately following the introduction of the heavy transient typically averages five cycles, which is shown in the graph of FIG. 2 at an 80 millisecond period at 87 V. The sub-transient period is followed by a transient period wherein the voltage output recovers to within a given tolerance of the target output voltage level. The sum of the two periods is usually termed recovery time. In many AVRs, the sub-transient voltage drop is in the range of 25-30% for a recovery time in the range of 250 to 500 milliseconds for a transient load demand ranging from 5% to 95% of the generator's rated capacity in kilo-volt-amperes (KVA).
A third common AVR design consideration is to maintain a constant Volt to Hertz (V/Hz) ratio in spite of frequency variations, thus avoiding saturation of the electromagnetic components of the load (e.g. transformers, motors, etc.). This characteristic should have an adjustable start frequency (i.e. the maximum frequency from which the V/Hz ratio starts, such as 60 Hz) and vary from the maximum operating frequency to a minimum that is the lowest expected frequency expected during an overload.
A fourth AVR design consideration is to provide a flexible V/Hz capability that can be adjusted so as to assist the recovery of the prime mover/exciter/generator combinations in case of a heavy transient overload. Flexibility means, in this case, to have the ability to: (A) adjust the V/Hz ratio from 1:1 to a higher value so that the output voltage will drop faster as the prime mover speed decreases as a result of a transient overload; and (B) set the frequency at which the V/Hz ratio begins to automatically adjust, such as a predetermined frequency below 60 Hz (usually between 59.5 Hz to 57 Hz) in order to provide regulation for light to moderate load changes.
The above four considerations are typically sufficient for stand-alone generator operation. If parallel operation is called for, then a fifth consideration called droop control may be required, which may be readily combined with some embodiments of the present invention.